1. Field of the Invention
The present invention relates generally to neurostimulation patches, and more specifically relates to external neurostimulation patches with multiple electrodes.
2. Description of the Prior Art
Neurostimulation is a process by which neurons are stimulated using artificial electrical pulses. A conventional method for neurostimulation includes the use of a pair of planar electrodes separated by a distance. Electric field lines from the pair of planar electrodes are used to stimulate the nerves. Generally, the two planar electrodes are of equal diameter and produce a low intensity electric field over a broad spatial area. Since this low intensity electric field is spread over a broad spatial area, it is less dependent on the exact location of the planar electrodes. The planar electrodes are fixed on the skin with adhesives, or can be implanted beneath the skin.
Currently, the available skin electrodes used for neurostimulation have one or more of the following limitations. Since the electric field of the skin electrodes is spread over a broad spatial area, it results in the electric field having lower intensity. Hence, more power is required to generate an electric field that can stimulate the nerve. Further, skin electrodes are fixed to the skin by means of adhesives. Prolonged ambulatory use of these skin electrodes causes skin irritation due to the adhesives used to fix them to the skin. Finally, because the specific nerve to be stimulated can only be stimulated over a very small area, precise placement of the skin electrodes is essential in order for the electrodes to be therapeutic.
In light of the foregoing discussion, there is a need for a device and method for precise and repeatable placement of skin electrodes, such that neurons surrounding the neuron to be stimulated are least affected. Accurate and repeatable placement of disposable electrodes by a patient outside of the caregiver's office is generally not possible, and this device and method will solve that problem. Further, there is a need for skin electrodes that cause minimal or no skin irritation during prolonged use. Additionally, there is a need for skin electrodes that require less power to stimulate neurons than the skin electrodes that are currently available.
The technology as described in U.S. patent application Ser. No. 11/146,522, filed on Jun. 7, 2005, having as named inventors Michael R. Tracey and Anthony DiUbaldi, and entitled “System and Method for Nerve Stimulation”, and further having been published on Dec. 15, 2005 and assigned Publication No. US 2005/0277998A1, the disclosure of which is incorporated herein by reference, is a non-invasive, transcutaneous neurostimulation device, which transmits a controlled, amplitude-modulated waveform. This waveform is composed of a carrier signal and a pulse envelope. The carrier waveform is designed to be of sufficient frequency to overcome attenuation due to tissue impedances. The pulse envelope contains specific pulse width, amplitude and shape information designed to stimulate specific nerves.
The patch is intended to be placed over the sacrum at the level of S2, S3 over the pudendal branch of the sacral nerve. Selectivity is accomplished by the design parameters of the waveform and the spatial location of the patch. That is, neurons of sufficient diameter closest to the patch will be stimulated and neurons of sufficient diameter furthest from the patch will not be stimulated. Therefore, it is critical for the patch to be applied in a location that is spatially closest to the target neuron. This dependency on spatial location can be somewhat mitigated by electrode design. That is, two equal diameter planar spaced electrodes produce a low intensity electric field over a broad spatial area. Conversely, concentric electrode geometry produces a high intensity electric field over a narrow spatial area. The broad electric field pattern will be more tolerant of placement location but will require more power to stimulate the target neuron. The concentric geometry will require less energy to stimulate the target neuron but will be less tolerant to placement error. Therefore, there is a need to design an electrode patch or placement tool that can repeatedly identify the ideal placement location.
U.S. Pat. No. 6,564,079, which issued to Philip C. Cory and Joan M. Cory, the disclosure of which is incorporated herein by reference, describes a sensor system comprising an electrode array and a skin attachment system for use with an electrical field-generating device that can non-invasively detect peripheral nerves and measure conductance at the skin surface. The Cory et al. patent does not, however, describe an array of equally spaced concentric electrodes to detect nerves deep in tissue that can repeatedly identify the ideal placement location.